/Engine/Source/Runtime/Engine/Classes/Kismet/KismetMathLibrary.h
Returns Slope Pitch and Roll angles in degrees based on the following information:
static void GetSlopeDegreeAngles ( const FVector & MyRightYAxis, const FVector & FloorNormal, const FVector & UpVector, float & OutSlopePitchDegreeAngle, float & OutSlopeRollDegreeAngle )